Cellular networks, particularly those specified by the Third Generation Partnership Project (3GPP) have developed the concept of a Self-Organising (or Optimising) Network (SON). These are also known as self-healing networks.
This concept allows the automated optimisation of network parameters, especially those of the Radio Access Network (RAN), in order to improve at least one Key Performance Indicator (KPI). The principles relating to SON functionality are specified in the Third Generation Partnership Project (3GPP) Technical Specification (TS) 36.300. Use cases are described in 3GPP Technical Report (TR) 36.902.
Most of the details for the SON algorithm is left to vendor implementation, but some interfacing specifications are standardised. For example, 3GPP TS 36.423 specifies the signaling over the LTE (Long Term Evolution) X2 interface to exchange or transfer parameters required by SON algorithms. Also, 3GPP TS 36.331 specifies parameters transferred over the LTE air interface by terminals for SON purposes. 3GPP have also specified the management framework for SON e.g. as detailed in TS 32.541; 3GPP TS 32.521; 3GPP TS 32.501; and 3GPP TS 28.628.
Although network operators have previously checked KPIs and scheduled optimisation activities in order to improve them, this has been a manual process in view of the complexity of the network and the relationship between the network parameters and KPIs. One such KPI is the Call Drop Rate (CDR). When this exceeds a certain value, it becomes a particular issue for network operators and requires immediate attention, due to the resultant customer dissatisfaction.
Various parameters affect the KPIs. The current focus of the SON concept relates to the optimisation of handover parameters to improve the robustness of handovers between cells of different base stations (for example, LTE eNodeBs). Improvement in handover performance has a direct impact on the call drop rate as a User Equipment (UE) moves between cells in the network. Handover optimisation using SON algorithms relies on parameters exchanged between neighbouring eNodeBs (eNBs) following a failure of the radio link. For example, this may take place immediately before the handover (too late handover), immediately after a successful handover (too early handover) or after reconnection to a cell other than the target cell where the handover occurred (handover to wrong cell).
The SON algorithm uses information exchanged on the X2 interface to decide on the need to change Radio Access Network (RAN) parameters. For example, this may include modifying the “time to trigger” parameters or hysteresis parameters. These may be indicated to a UE using broadcast signaling or by dedicated means. In this way, an SON algorithm can improve the success rate of subsequent handovers between the optimised cells and thus improve the overall network KPI. For existing SON algorithms to perform effectively, the handover failures should be the result of the non-optimised RAN parameters, rather than any external factors that cannot be improved by changing these RAN parameters.
One such external factor that can result in poor handover performance is the reliability of the backhaul (or backbone) interface between base stations, for instance those over which the X2 protocol for handover runs between eNBs in LTE. Backhaul interfaces are used for communication by each base station with other network entities of the cellular radio network. Typically, the transport link between base stations uses microwave communication which has a certain element of unreliability due to varying radio conditions.
Moreover, US-2012/157089 observes that the network KPIs may be adversely impacted when the performance of backhaul interfaces degrades or fails. They further observe that SON mechanisms, such as cell outage compensation, can compensate for such degradation or failure by adjustment of the RAN parameters and therefore the degradation or failure can go unnoticed for a long period of time. To avoid backhaul failures going undetected, US-2012/157089 suggests methods for identifying these problems using a Network Management System (NMS). Automatic detection of an outage, high error rate or both, due to backhaul failures, can then be performed and these issues can be resolved independently from the SON compensation processes in the RAN, as needed.
Even if this technique could identify and resolve backhaul failure issues automatically, it could only address the case where there are permanent failures in the backhaul connection. For example, these may be caused by faulty hardware or obstruction in the radio path. However, it could not address the problem caused by intermittent degradation of the backhaul connection quality.